#!/usr/bin/python3

aaa = input("please input a num:")
if  aaa.isdigit():
    grade = int(aaa)
    if 90 <= grade <= 100:
        print("A")
    elif 80 <= grade <= 89:
        print("B")
    elif 70 <= grade <= 79:
        print("C")
    elif 0 <= grade <= 69:
        print("D")
else:
    print("please input number between 0-100")
